A basic function of a distributed data communications network is to transmit data between any two points in the network. The routing of this data should be accomplished in the most efficient manner possible. A distributed data communications network is a hierarchical system of hardware components arranged so that each hardware device is connected, directly or indirectly, to every other device. At the lowest level are user terminals or hosts, which form part of the local access network. These terminals are linked to one or more concentrators, which are statistical multiplexers with several low data rate output data lines and a fewer number of high data rate output data lines. The concentrators form the second level of the hierarchy and, together with the terminals, form the access network.
The concentrators, which may be connected to other concentrators in a hierarchical fashion, are ultimately connected to the backbone, which forms the highest level in the hierarchy. The backbone consists of high speed, high capacity links that terminate at backbone nodes. A backbone node consists of one or more devices, and includes at least one switching device for routing traffic within the backbone. The traffic is the quantity of data per unit time that is transmitted through the lines. The traffic from the concentrators enters the backbone at a backbone node.
A collection of terminals and associated concentrators are aggregated into a cluster along with one or more switches at a single backbone node. More than one cluster may be associated with a single backbone node, since there may be a plurality of switches at each backbone node location.